The most terrifying sight on television this evening may be that of a tiny metal pellet barely visible to the naked eye. Shown under a microscope, it looks like the skull of an unspeakably eerie alien, featureless but for what appear to be two hollow eye sockets.

On September 7, 1978, the pellet was either injected or shot, via the point of a KGB-designed umbrella, into the right thigh of the Bulgarian defector and dissident author Georgi Markov as he waited for a bus on Waterloo Bridge in London. The “sockets” were laser-drilled holes, sealed with wax, containing minute drops of ricin, a lethal poison derived from the castor oil plant. Once inside Markov’s bloodstream, the wax melted and the toxin spread slowly through his body, shutting down his organs until he died of heart failure four days later on September 11.

The sensational nature of this piece of Cold War “cloak-and-umbrella” skullduggery, still officially unsolved, is revisited tonight in a one-hour documentary, “Umbrella Assassin,” as part of the PBS series Secrets of the Dead.

Although the name “James Bond” crops up at least twice, one of the lessons “Umbrella Assassin” teaches is the discrepancy between real-life Cold War intrigue and its on-screen glamorization. When he went to a hospital eight hours after the attack, complaining of sickness and fever, Markov was treated as a paranoiac. Though a well-known dissident who broadcast scathing criticism of Bulgaria’s communist government over Radio Free Europe, he was considered delusional by the physicians on duty.

Markov died an agonizing death, vomiting blood as his organs hemorrhaged. Having no clue as to the cause (the pellet was discovered during an autopsy), the doctors were powerless to help. Ricin, a poison of great interest to terrorists, has no known antidote. That, and the date of Markov’s death, gives “Umbrella Assassin” an added contemporary resonance.

It doesn’t, however, make it an entirely satisfying documentary. Leaning heavily on the research of a British journalist, Jack Hamilton, who is interviewed at unimpressive length, the film is least persuasive when it claims to have finally identified the killer, a Copenhagen-based Italian named Francesco Giullino employed by the Bulgarian secret service during the Cold War, whom neither the Danish, Bulgarian, nor British authorities have bothered to arrest. In the film, Mr. Giullino is described by Mr. Hamilton’s primary source in Copenhagen “as a little man with a small mustache,” yet in an article Mr. Hamilton wrote for the Times of London last year, he was described as “heavily built.” So which is it?

“Umbrella Assassin” makes free use of the kind of dramatic “re-enactments” that were once frowned upon. The most crucial is that of the assassination itself, which is shown in two different ways. In one version, the killer (who doesn’t look “little”) uses an umbrella; in the other, some sort of pen. (It’s not certain that Markov was killed with an umbrella, only that his assassin was carrying one.) According to Mr. Hamilton’s article, Markov “felt a stinging pain in his thigh as he mingled with rush-hour commuters waiting for a bus on Waterloo Bridge.”

Yet, as presented by the filmmakers, this “rush hour” is so sparse as to suggest that the entire population of London must have been given the pink slip. As for mingling commuters, there are precisely two people at the bus stop — Markov and a woman in front of him — and less car- or foot traffic than at 4 p.m. on Christmas Day.

A dark-haired man with a mustache appears, carrying an umbrella. A moment later (version one), he glancingly stabs Markov’s leg with the skill of a fencer, drops the umbrella, picks it up again, apologizes, and then hails what appears to be the only taxi in London and disappears inside it. In version two he stands immediately behind Markov and shoves the pen into his thigh. The latter action seems far too obvious and brutal to be plausible, while the former would seem more credible had it occurred in a crowd. Either way, fans of Cold War intrigue will find it anticlimactic.

“Umbrella Assassin” is on firmer ground when it takes us inside Porton Down, the British chemical warfare facility where the pellet was first examined and ricin was determined to have been the poison used. There is also an interview with a Bulgarian dissident who survived a similar attack, along with interviews with various doctors, pathologists, intelligence experts, and the Bulgarian investigator who claims to have found the assassin’s name in the few remaining secret government files that weren’t destroyed. That weaponized KGB umbrellas did exist is amply proven by a collector in Florida who displays several and explains how they worked.

Whoever killed Markov, the hit was almost certainly ordered by Bulgaria’s then-president, Todor Zhivkov, both as revenge on a defector — Markov was once a favorite of the regime — and as a warning to others. It was also a morbid tribute to his own political potency. September 7, the date of the attack, was the Bulgarian dictator’s birthday. Black-and-white footage of Zhivkov with Leonid Brezhnev at various party congresses, where the smallest remark leads to enthusiastic applause, and establishes the banal horror of life behind the Iron Curtain.

“The death of one man is a tragedy, the death of millions is a statistic,” Joseph Stalin famously said. Markov remains a haunting reminder of how a determined dictatorship can reach across borders to bring death and headlines to one man while countless others die to near-universal silence. The good news is that while it’s easy to kill a single person with ricin, the toxin’s volatility would make it extremely hard for someone to use it to kill millions.